The present invention relates to a plural oscillator microwave frequency system in which at least two oscillators of distinct different frequencies have the outputs combined and applied in common to a coaxial type transmission line and, more particularly, to a novel microwave subsystem in which the outputs of two oscillators of different frequency are diplexed without significant interactive interference between the oscillators.
Various microwave systems, such as radars and electronic countermeasure equipments, known to those skilled in the art, employ one or more microwave frequency sources or oscillators, as variously termed, to provide electromagnetic signals in the microwave frequency region for various applications. In some systems, I am advised, there is need to combine or couple the outputs of two oscillators operating at different frequencies, designated generally as F.sub.(1) and F.sub.(2), to a common coaxial type transmission line over which the signals may propagate to other circuits, not here relevant, within the microwave system. Either one or the other of such oscillators may be activated to provide signals of frequency F.sub.(1) or F.sub.(2), respectively, or both of such oscillators are simultaneously activated to provide an output signal of frequency F.sub.(1) and F.sub.(2). Simple as that may seem to the lay person, the goal has not been, in my opinion, whether or not signal combining may be accomplished but whether or not such combining can be accomplished by a simple structure without exceptional losses of microwave frequency power and without interactive interference between the oscillators, such as where one oscillator could become an electrical load to the other oscillator causing power losses or possible damage or both. Device damage is particularly undesirable in the case of modern solid state IMPATT diode type or FET type oscillators, containing the sensitive and easily damaged IMPATT diode or FET as the microwave frequency generating devices, although that is not a problem, in my opinion, with Gunn diode type solid state oscillators with which the invention is also used. I believe that prior apparatus exists by means of which separate oscillators may have their outputs combined for application to a single coaxial type transmission line. These I refer to by the common names, understood by those skilled in the art, as a hybrid combiner, a "rat race" circulator, and a "hybrid T". As those skilled in the art of these devices may attest, such devices, although suitable for the purpose, are either very complex to manufacture and adjust or are large and bulky, or involve a combination of both problems, in my opinion.
Another prior art structure made known to the applicant appears in the triplexing device presented in U.S. Pat. No. 2,909,774 to deBell. That device includes at least two oscillators coupled to two sections of coaxial line which are then joined to a third tri-coaxial line. The two oscillators are specified to bear a frequency relationship to one another in a multiple, and incorporates four coaxial stub tuners, two of which are located proximate the oscillator end of the associated coaxial line section of one oscillator and two proximate the other oscillator. One tuner adjacent the first oscillator is tuned together with the associated coaxial line section so that the input to the associated coaxial line section presents a high impedance at the frequency of the second oscillator, as well as a short circuit to energy of the second oscillator at the end of the coaxial line section to where the first oscillator is coupled, and vice-versa in the case of the coaxial tuning stub associated with the first oscillator. The remaining two stubs are intended to adjust the line to cancel the effect of the associated tuning stubs. In the deBell structure, open circuit tuning stubs are employed which would appear to allow microwave radiation to escape to the surrounding ambient, possibly causing interference. Also those skilled in the art recognize the "narrow-band" characteristic and sensitivity of tuning stub devices, which encourages misoperation, particularly if either oscillator drifts in frequency or is modulated as might defeat the intended operation of the deBell device, and recognize as well the overall complexity and awkwardness of the mechanical structure. Although complexity of adjustment and manufacture is to be avoided, bulkiness is particularly undesired in modern day airborne radar and countermeasure systems in which space and size are at a premium and in which one usually wishes to obtain the size reductions permissible with the modern solid state semiconductor oscillators which in size may be no more than 6 inches .times. 5 inches .times. 4 inches in dimension.
A prime object of my invention is to provide microwave apparatus which satisfactorily combines the outputs of two oscillators of distinct frequency to a common output in a relatively uncomplicated novel arrangement of reduced overall size.